The present invention relates to an image projection apparatus, such as a liquid crystal projector, which is provided with a cooling fan.
Projector is provided with heat-generating parts which are optical system parts including a light source, light modulation elements such as liquid crystal panels and optical elements, and electrical system parts including a light source ballast and a CPU.
To appropriately cool these heat-generating parts, the cooling method disclosed in Japanese Patent Laid-Open No. 2000-19496 uses plural fans. In this method, plural flow paths are formed in accordance with heat values of the heat-generating parts, such as a flow path for cooling a light source, a flow path for cooling a light modulation element and a flow path for cooling a light source ballast. The cooling airflows that passed through the flow paths are finally combined and exhausted by a fan.
This achieves cooling with high efficiency by using the minimum number of fans to reduce the size of the projector. Furthermore, this can decrease the number of exhaust openings through which noise generated inside the apparatus leaks. This may achieve a low-noise projector.
However, in such a configuration in which plural airflows flow into one fan from plural flow paths, when the plural airflows have greatly different flow velocities, noise generated in the fan is increased.
This phenomenon will be described using FIG. 7. FIG. 7 shows the phenomenon of generation of noise in an axial flow fan.
In this figure, reference numeral 18F denotes the axial flow fan. Reference symbol WA denotes an airflow which is taken in into the axial flow fan 18F from a first flow path. Reference symbol WB denotes an airflow which is taken in into the axial flow fan 18F from a second flow path. Reference numeral 18Fa denotes the leading edge of a blade of the axial flow fan 18F in its rotation direction.
The airflow WB has a velocity much higher than that of the airflow WA. In this case, the leading edge 18Fa of the blade perpendicularly cuts each airflow when the fan 18F is rotated. The leading edge 18Fa of the blade hits the side face of the airflow WB when the leading edge 18Fa that was cutting the slow airflow WA starts to cut the fast airflow WB, which generates wind noise. This wind noise is peaked at a frequency of an integral multiple of ‘the number of the blades x the rotation speed thereof’, which causes noise.
In addition, the leading edge 18Fa of the rotating blade forms different angles of attack with respect to the airflows WA and WB which have different velocities. The airflow WB providing a larger angle of attack becomes burble on the surface of the blade, which causes turbulent flow noise.
A conventional art is known, as disclosed in Japanese Patent Laid-Open No. H11-82393, in which an airflow is uniformly introduced to a fan to allow the airflow to be uniformly blown to a heat-generating part, thereby carrying out cooling with good efficiency.
The art disclosed in Japanese Patent Laid-Open No. H11-82393, however, is an art for a case of a single airflow and an art focused on the flow volume distribution on the blowout side of the fan. In other words, the art does not aim to reduce noise when plural airflows are taken in into one fan.